A bit of a wide open door there...
First thing to do is decide what you want the final product to be/look like. Do you want it stock looking but with all modern driveline or do you want it to a full on restomod with some aero added, tall wheels with skinny tires, or maybe the "field fresh" look but with modern things inside or perhaps you want to go for the retro 70's street brawler look with big meats out back, forward rake and skinnies up front or something in between all of this. You don't have to decide on the "exact" blueprint but you should have an idea because it will affect some of your choices as you move forward and "choices" in this game equates to dollars, usually a lot of them.
All that said here are some "basic" ideas that should apply to all of the potential build styles;
Get the sub structure of the car solid (floors specifically) so you have a solid foundation to build from. The body can be done later if you like and this is usually something that traps builds for decades (body work). I would build the car and do the body work last unless you have deep pockets from the beginning but understand that the car will need to be "built" several times since you are wanting to modify it.
You will want to look at strengthening the car with the addition of sub frame connectors, torque boxes, etc. Even the old school drive lines require this IMO, makes driving them significantly better and safer.
A modern drive train is not that much different than the old stuff, yes there are more electronics but that is not an issue. Going to a G3 you will need to decide on;
Motor mounts - get them from the same people as you headers
Headers - quite a few out there but I prefer TTI
Accessory drive kit - you can reuse the factory stuff or go with something like Holley offers (expensive but nice)
You will need a new fuel system, one that offers a return line - Tanks Inc should have one for that car. You will also need an in tank electric fuel pump, Tanks also has that along with the fuel sender.
Install a new wiring harness - quite a few to select from, I like Kwik Wire myself. All of the wires are clearly labeled as to where they go.
You will need a Engine Control Unit (ECU) - you can use a factory system that has been re-flashed or go with something like a Holley Terminator X.
Transmission - if you go with a modern AT like a NAG you will have to modify the floor a fair bit. USCT sells a kit to do just that. You could also go with a factory A833 4 spd or something like a TKX, all of the parts/pedals are available.
Brakes are anything from bone stock (disc/drums, drum/drum) to upgraded systems such as a 4 wheel disc system. I did a lot of research on this issue and I would recommend retaining rebuilt stock pieces unless you have to have something else. Yes the Wilwood brakes "look" good but unless you are going to 6 piston I don't think you are getting a lot of improvement. That said Baer makes some good kits and is worth consideration.
Lots of info on here and on YT, have fun but do yourself a favor and plan it out some first.
